Organic electrolyte batteries

ABSTRACT

Corrosion of the current collectors in cells utilizing vanadium oxides and chromates as cathode materials is prevented by the utilization of metals from Groups IVb, Vb, and VIb of the Periodic Table for fabricating the current collectors and leads in contact with the cathode materials. Ta, Ti, and Mo are preferred materials. Cells containing such structures are described as well as an intermediate protective structure.

This is a division of application Ser. No. 503,821, filed Sept. 6, 1974,now U.S. Pat. No. 3,945,852, issued on Mar. 23, 1976.

FIELD OF THE INVENTION

This invention relates to high energy density organic electrolyte cellsbased upon light metal anodes and strongly oxidizing cathodes and moreparticularly to specifically useful structures and structural materialsfor cathodes utilizing vanadium oxides, and metal chromates ordichromates.

BACKGROUND OF THE INVENTION

The Li/V₂ O₅ (Ser. No. 829,849, now abandoned) and Li/metal chromate(U.S. Pat. No. 3,658,592) organic electrolyte cells show excellentenergy density and storability at ambient temperatures. However, atelevated temperatures (e.g. 55° C or higher) the cells exhibit corrosionof the nickel cathode current collector.

1. The Li/V₂ O₅ organic electrolyte cells made according to theprocedure described in Ser. No. 829,849, showed severe corrosion of thenickel current collector and the nickel tab to the degree that the tabbecame disconnected from the current collector after a storage of threemonths at 55° C. The corrosion was particularly severe at the point werethe nickel tab was welded onto the expanded nickel current collector.This resulted in a dead cell due to the lack of electrical contactbetween the cathode terminal and the cathode current collector.

OBJECTS OF THE INVENTION

a. To eliminate the problem of corrosion of cathode current collectors,and

B. TO IMPROVE THE ELEVATED TEMPERATURE STORABILITY OF ORGANICELECTROLYTE BATTERIES.

THE INVENTION

I have discovered that it is possible to protect nickel currentcollector tabs for cathode assemblies to permit an extension of thecorrosion resistant life of cathodes beyond the three month, 55° Cstorage, maximum life expectancy of unprotected nickel currentcollectors. My method for protecting the current collector tabs in theweld areas which are most prone to corrosion consists of heat sealinglayers of polyolefinic polymer foils on both sides of the tab weld. Theheat-sealed foils extend completely around the tab-weld area and thusprevent the access and contact of electrolyte and cathode activematerial to the corrosion-prone tab-weld area.

The details of this aspect of my invention are illustrated in thedrawing wherein:

FIG. 1 shows the cathode assembly current collecting grid exposed in thearea of the tab-weld;

FIG. 2 shows the cathode assembly of FIG. 1 with the nickel tab weldedin place to the current collector;

FIG. 3 shows the cathode tab-weld area of FIG. 2 protected by heatsealed polyethylene foils; and

FIG. 4 is an enlarged view along line 4--4 of FIG. 3 showing thetab-weld area detailing the protection of the tab-weld area bypolyethylene foils, heat sealed to the grid and embedded in the cathodeactive material. The procedure according to this aspect of the inventionresult in cells which survived the accelerated test of 3 months ofstorage with the electrolyte in contact with the cathode.

However, I have found that cathodes so constructed, showed tab andcurrent collector corrosion after 6 months of storage at 55° C showingthat the problem, while successfully ameliorated, still persisted.

My original solution to the corrosion problem was directed to thetab-weld area of the current collector assembly where the nickel currentcollector embedded in the cathode active material was fastened to thenickel tab leading from the cathode to the cell terminal connections. Ithad been noted that this was the focus of the corrosion when the cellswere stored under accelerated aging conditions.

Welding conditions were varied, corrosive resistant alloys were used forthe tabs, the welded areas as well as the current collectors and tabswere plated with protective, yet highly conductive, metals such as gold,but none of these expedients imparted sufficient corrosion protection towarrant the effort and expense. Then I discovered that by protecting themost corrosion-prone area, the tab-weld area, from access by theelectrolyte by heat sealing electrolyte-proof foils around thesecritical areas, I could make cells that uniformly survived storage at55° C for 3 months. Few of the previous expedients survived this testand none did so uniformly. The heat-sealed foils of this invention arefoils of polyolefin polymer materials such as polyethylene andpolypropylene.

Heat-sealable films of such materials are commercially available invarious thicknesses ranging from 0.001 to 0.010 inch in thickness. Ihave found 0.002 inch thickness of polyethylene to be satisfactory andpreferred as it is easily heat sealed around the tab-weld area. Suchseals uniformly and adequately protect this critical corrosion pronearea. The heat seals to the metal are easily fabricated with light,localized pressure by heated platens.

The current collectors of this aspect of this invention are fabricatedby welding the nickel tabs 4, to Exmet grids 3. Then the areas of theweld 5 are overlain on each side of the weld by 2 mil polyethylene foils6 and heat sealed by platens at the temperatures recommended for thespecific brand of foils.

After this protection treatment, the cathode-active material 2 isapplied to the current collector assembly by the techniques described inU.S. Pat. No. 3,658,592 and Ser. No. 829,849 to form cathode 1.

A variant method for applying the cathode active material 2 from themethod described in U.S. Pat. No. 3,658,592 is that the chromatecathodes are dried at 60° C in a vacuum instead of at 300° C atatmospheric pressure as previously described.

I have further discovered that metals of Group IVb, Vb, and VIb of thePeriodic Table do not corrode when in contact with cathodic oxidizingagents such as vanadium pentoxide (V₂ O₅), mercury (II) chromate(HgCrO₄), and silver chromate (Ag₂ CrO₄), even at elevated temperatures.

Further, I have discovered that cathode collectors of tantalum (Ta),titanium (Ti), molybdenum (Mo), zirconium, (Zr), niobium (Nb), vanadium(V), chromium (Cr) and tungsten (W) or mixtures thereof in contact withsuch cathode-active materials are useful in the manufacture of cathodesfor electrochemical cells based upon lithium or other light active metalanodes, organic electrolytes and such cathodes.

Such cells have extended shelf life when stored at 55° C beyondcomparable cells manufactured with such conventional "anticorrosive"current collectors as nickel, silver, stainless steel, Inconel,gold-plated nickel, and polyolefin protected nickel.

It is a theory that the unique corrosion resistant properties of themetals of Group IVb, Vb, and VIb of the Periodic Table particularly Ta,Ti, and Mo which are preferred, are derived from their property offorming protective oxide films upon contact with these oxidants in thepresence of the organic electrolytes. These oxide films, whileprotective of the underlying metal, possess a high degree ofconductivity. Such cells show no increase in internal resistance whencompared to cells using conventional current collectors.

DETAILS OF THE INVENTION

The V₂ O₅, HgCrO₄ and Ag₂ CrO₄ cathodes, made in a manner similar tothat described in the application Ser. No. 829,849 and U.S. Pat. No.3,658,592, on expanded-metal substrates such as Ni, Inconel, gold platedNi, Ag, Ta, Ti, Mo, Zr, Nb, V, Cr, W, and stainless steel, were refluxedat 85° C in an electrolyte consisting of 1M LiClO₄ in an equivolumemixture of tetrahydrofuran and propylene carbonate, in order toaccelerate the corrosion phenomenon. The reflux temperature was used toaccelerate the corrosion test.

At the end of 233 hours of refluxing, the cathodes were examined. It wasfound that the expanded metal grids of Ta, Zr, Nb, V, Cr, W, Ti, and Modid not show any signs of corrosion, whereas all the other metals werepartially or completely corroded.

Of the protected non-corroding metal grids and tabs, tantalum, titaniumand molydenum are preferred because of the ease of manufacture, lowercost and commercial availability.

The elevated temperature stability of the Li/V₂ O₅, Li/HgCrO₄ and Li/Ag₂CrO₄ organic electrolyte cells were thus improved by the use of cathodegrids and tabs made of metals such as Ta, Ti and Mo. The metals such asZr, Nb, V, Cr, and W also improve the elevated temperature storabilityof the cells when used for cathode grids and tabs.

These cathode current collector materials may be used in the form ofexpanded metal sheets, wires, flat tabs, and cathode cups. The activematerial may be packed around the expanded metal, may have screens ofwire of these metals included therein, or the sheets may be fashionedinto terminal tabs or cathode cups as long as there is conductivecontact between the cathode current collector and the cathode activematerial.

The current collectors, while particularly effective for vanadium oxide,and metal chromate and dichromate cathodes in organic electrolytes, mayalso be used for milder cathode active materials such as other metaloxides, halides, permanganates, arsenates, periodates, vanadates,persulfates, sulfites and mixtures thereof. The metal chromates includechromates of lead, iron, cobalt, nickel, thallium, bismuth and mixturesthereof and dichromates as well as chromates of silver, mercury andcopper.

The organic electrolyte includes combination of electroylte saltscompatible with the electrodes of the cells, and solvents includingelectrolyte salts such as perchlorate, hexaflurorphosphate,tetrafluoroborate, or hexafluoroarsenate salts of lithium or other lightmetal.

Solvents such as propylene carbonate, tetrahydrofuran, dimethoxy ethane,methyl formate, acetonitrile, dimethyl carbonate, dimethyl sulfoxide,dimethyl sulfite, dimethyl formamide, gammabutyrolactone, andN-nitrosodimethylamine and mixtures thereof are included.

Anodes for such cells include Li, Na, K, Ca, Mg, and Al.

What is claimed is:
 1. A high energy density cell comprising an anodeelectrode of an active light metal selected from the group consisting oflithium, sodium, potassium, magnesium, calcium, beryllium, and aluminum;an electrolyte salt of said light metals; an organic solvent havingdissolved therein said electrolyte salt; an active cathode materialselected from the group consisting of metal chromates, dichromates,oxides, halides, permanganates, arsenates, periodates, vanadates,persulfates, sulfites and mixtures thereof; and a cathode currentcollector in direct contact with said active cathode material; saidcurrent collector being composed of a material selected from the groupconsisting of the metals of Groups IVb, Vb, and VIb of the PeriodicTable and mixtures thereof.
 2. A cell according to claim 1 wherein saidelectrolyte salt is selected from the group cnsisting of perchlorate,hexafluorophosphate, tetrafluoroborate, and hexafluoroarsenate salts ofsaid light metals and mixtures thereof.
 3. The cell according to claim 1wherein said organic solvent is selected from the group consisting oftetrahydrofuran, dimethoxy ethane, methyl formate, acetonitrile,dimethyl carbonate, dimethyl sulfoxide, dimethyl sulfite, dimethylformamide, gammabutyrolactone, N-nitrosodimethylamine and mixturesthereof.
 4. A cell according to claim 1 wherein said active cathodecomprises vanadium pentoxide, or a chromate or dichromate of silver,mercury, copper, lead, iron, cobalt, nickel, thallium, bismuth or amixture thereof.
 5. A cell according to claim 1 wherein said cathodecurrent collector comprises titanium, tantalum, or molybdenum.